1. Field of the Invention
The invention relates to a lens, and more particularly relates to a fixed focal length lens.
2. Description of Related Art
Design of a projection lens is a challenge for an optical lens designer. The projection lens with a good performance may have advantages of high image quality, low distortion aberration, high resolution, high contrast, and uniform image illumination. Moreover, to achieve a function of projecting a large image within a short distance, the projection lens may have a large field of view (FOV) and a relatively low distortion aberration. To increase a light efficiency and the illumination uniformity of a projected image, a maximum angle (i.e. a telecentric angle) of a chief beam at a reduced side of the projection lens relative to an optical axis is designed to be relatively small, so that the chief beam is approximately parallel to the optical axis.
The above design requirements are impeded by each other, so that a design difficulty of the projection lens is increased. For example, when the distortion aberration is reduced as far as possible, the FOV of the projection lens and the number of the lenses used by the projection lens are influenced. If the projection lens is designed to have a large FOV, and the chief beam at the reduced side is approximately parallel to the optical axis, a length and a size of the projection lens are liable to be increased. Moreover, to achieve the advantages of high imaging quality, low distortion aberration, high resolution, high contrast, and uniform image illumination, aspheric lenses are generally used though application of the glass aspheric lenses may increase a cost, so that the cost of the projection lens may not be reduced. If spherical lenses are used for a projection lens merely, the number of the lenses used by the projection lens may be over and above 9, so that the size of the projection lens is increased, and the cost of the projection lens may not be effectively reduced.
U.S. Pat. No. 5,042,929 discloses a lens set having 7 lenses. The lens set includes a front lens group and a back lens group. The front lens group includes 6 lenses, and refractive powers of the lenses from a magnified side towards a reduced side are respectively positive, negative, positive, negative, positive, and positive. A back focal length of the front lens group is longer, and thus a length of the whole lens set is longer as well, which is hard to miniaturize the lens set. The back lens group has only one positive lens, and the positive lens is a plastic aspheric lens. Since the plastic aspheric lens is close to the reduced side, it is difficult to assemble. When the lens set is being focused, the back lens group is fixed, and the front lens group is movable. In other words, a focusing mode of the lens is not an internal focusing type, and an emitting light is liable to be interfered with other mechanical members, so that a shade angle problem of an imaged picture occurs. To resolve such problem, the mechanical members are disposed apart from the lens set as far as possible, so that the size of the whole lens set is increased. In addition, since the number of the glass lenses used by the lens set is 6, a lightweight of the lens may not be achieved, and a cost of the lens set may not be reduced.
U.S. Pat. No. 7,173,777 discloses a lens set having 8 lenses grouped into a first lens group, a second lens group, and a third lens group. The first lens group has a negative refractive power and includes two lenses, wherein at least one of the lenses is a plastic aspheric lens. The second lens group has a negative refractive power and includes one lens. The third lens group has a positive refractive power, and includes five lenses. The number of the glass lenses of the lens set is relatively more, so that a lightweight of the lens set may not be achieved, and a cost of the lens may not be reduced.
Taiwan Patent No. M309675 discloses a zoom lens, wherein the zoom lens includes a first lens set having a negative refractive power, a second lens set having a positive refractive power, and a third lens set having a positive refractive power arranged in sequence from an object side towards an image side. U.S. Pat. No. 6,124,984 discloses a zoom lens, wherein the zoom lens includes a first lens set having a negative refractive power, a second lens set having a positive refractive power, and a third lens set having a positive refractive power. When the zoom lens is zoomed from a wide-end to a tele-end, the first lens set and the second lens set are required to be moved for reducing a distance between the first lens set and the second lens set without increasing a distance between the second lens set and the third lens set, so as to implement a 2.5 zoom ratio performance.
U.S. Pat. No. 6,124,987 discloses a zoom lens including a front lens set having a negative refractive power and a back lens set having a positive refractive power. The first surface of the first lens in the front lens set is an aspheric surface, and the second lens in the back lens set also applies the aspheric design. When the zoom lens is being zoomed, the front and the back lens sets are relatively moved along the optical axis. When the zoom lens is being focused, only the last lens having a positive refractive power in the back lens set is moved. U.S. Pat. No. 7,453,651 discloses a zoom lens including a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. U.S. Pat. No. 6,844,984 discloses a lens including a first lens group, a second lens group, and a third lens group. The first lens group is used to focus, the second lens group is used control zooming, and the third lens group is fixed. U.S. Pat. No. 5,550,679 discloses a projection lens including a first lens set having a negative refractive power, a second lens set having a positive refractive power, and a third lens group having a positive refractive power.